1. Field of the Invention
This invention relates generally to an improved self-propelled grass mowing machine of the tri-plex type having a plurality of cutting units (e.g., three cutting units). Such a machine includes a powered body or traction unit which carries an operator who drives and controls the machine. More particularly, this invention relates to the pivot structure for the steerable drive wheel of such a mowing machine.
2. Description of the Prior Art
Grass is a widely used ground cover for both private and public property. However, grass covered areas are sometimes very large which presents difficult problems in keeping the grass properly mowed. It is not uncommon for cemeteries, golf courses, parks, highway median strips, and the like to extend over many acres of land and to be wholly or partially covered by grass. Such large areas usually are not capable of being efficiently mowed by "small scale" or household grass cutting equipment, such as a hand pushed walk-behind lawn mower. Furthermore, at least some of these large grass covered areas must be frequently mowed. For example, the grass on a golf course or cemetery must be mowed as often as once a week or more, especially during the height of the grass growing season. This further dictates against the use of small scale lawn mowing equipment. If such equipment were used to cut such large areas, it would require a large number of lawn mowing machines and a relatively large labor force to keep the grass adequately mowed. This would make the cutting of these grassy areas extremely expensive.
Various self-propelled "large scale" mowing machines have been designed to more efficiently cut large areas of land covered by grass. One general type of such a mowing machine is that known in the industry as a tri-plex machine. The tri-plex machine is a self-propelled mowing machine of the rider type (i.e., one which carries an operator) having an internal combustion engine as the basic power source. The machine comprises a frame or body member which is movably supported by a plurality of drive wheels. Three drive wheels arranged in a triangular relationship are sometimes used. In addition, the body of the tri-plex mowing machine usually supports three cutting units which are vertically movable into and out of engagement with the ground. Each of the cutting units comprises a rotatable reel type blade which sweeps the grass against a fixed bed knife to cut the grass. The drive for the cutting units and the drive wheels is usually taken off the internal combustion engine by generally conventional power transmission systems (e.g., mechanical belt drives and the like).
When a three wheeled arrangement is used to support the body of the grass mowing machine, the unopposed third drive wheel (e.g., the rear drive wheel) is often a steerable drive wheel. Referring to FIGS. 9 and 10, it is conventional to support the rear drive wheel 100 of the mowing machine in a clevice support block 101. Clevice block 101 is U-shaped having two opposed side plates 102 and 103 and a top plate 104. The opposed side plates 102 and 103 rotatably receive the axle 105 of the drive wheel 100. Top plate 104 carries an upwardly extending cylindrical spindle 106. Spindle 106 is rotatably received in a sleeve rigidly mounted on the body of the mowing machine to journal the drive wheel for rotation about a vertical axis. This type of support arrangement for the steerable drive wheel 100 is generally similar to the support arrangements of conventional caster wheels on various types of commonly known household goods, e.g. the caster wheels used to movably mount furniture and the like. Thus, the prior art method of supporting the rear drive wheel in a mowing machine will be generally referred to as the caster method and the steerable rear drive wheel 100 as the caster wheel.
Although the use of a caster wheel as the steerable drive wheel adequately supports the mowing machine in most situations, certain problems are nonetheless presented by this type of support arrangement. Spindle 106 extends vertically relative to the top plate 104 of the clevice block 101. In addition, the side plates 102 and 103 of the clevice block 101 must be long enough such that the top plate 104 itself clears the upper surface of the drive wheel 100. Thus, the height above ground level of the clevice block 108 and the attached spindle 106, which will be referred to as y.sub.1 for clevice block 101, is often quite large. This requires that the body of the mowing machine have a hump or otherwise be suitably enlarged to accomodate the vertical height y.sub.1 of the drive wheel support apparatus. This increase in the height of the mowing machine means that extra material is needed to manufacture the machine, thereby increasing its manufacturing costs and purchase price. In addition, the use of a hump on the body or an enlarged body frame to enclose the wheel support apparatus detracts from a sleek profile of the mowing machine and to at least some people is not aesthetically pleasing.
Another problem often associated with caster type drive wheels 100 is their ability to self correct in only one direction of movement of the mowing machine. This is best illustrated when considering the movement of the mowing machine into and through a turn. When the operator of the mowing machine desires to change the direction of movement of the machine, he turns the steering wheel which rotates the steerable drive wheel 100 about the axis of the support spindle 106. After the turn is completed, the operator releases the steering wheel. It is desired at that time that the drive wheel 100 self correct or move back to its normal or straight ahead position. When the mowing machine is being driven in a forward direction, most prior art caster wheels 100 will self correct to the desired position. However, when the machine is being in a reverse direction, caster wheels 100 often do not self correct. Instead, the caster wheels will remain rotated from their normal position which causes the mowing machine to keep turning even after the steering wheel is released. In some instances, the caster wheels may even rotate further about the support spindle to increase the turn of the mowing machine.
The inability of a caster type steerable drive wheel to self correct when the mowing machine or vehicle is being driven in a reverse direction is generally disadvantageous. For one thing, it does not allow proper operation of the mowing machine in the reverse direction. This decreases the flexibility of the mowing machine. In addition, it may prohibit the operator from backing the machine into restricted spaces because of the operator's inability to precisely control the movement of the vehicle in a reverse direction due to the failure of the steerable drive wheel to self-correct in this direction.